Pickup tubes



Jan. 13, 1959 w, G. SHEPHERD 2,869,016

PICKUP TUBES Fuga Aug. 2, 1956 ATTORNEY Zhhi .Patented dan. i3, i959 ine PICKUP TUBES Wiliiam Gerald Shepherd, St. Pani, Minn., assigner to Bendix Aviation Corporation, Eatentown, N. d., a cor poration of Deiaware Application August 2, 1956, Serial No. 691,726 1. Claim. (Cl. 313--339) The present invention relates to pick up tubes or" the im age orthicon type and more sensitivity thereof.

Heretofore the image orthicon has been limited in sensitivity at low light levels by the shot noise in the scanning beam. Light signals below a certain point produce a response which is insutlicient to override the spurious signal caused by the shot noise in the beam. ln other Words the noise signal is stronger than the light signal, This spurious signal is a result of using a limited aperture to form the beam. A small aperture is tlooded by a beam of much larger diameter. Random interception of electrons by the edge of the aperture produce these spurious signals.

The present invention provides particularly to improving the means for producing a tine pencil beam of electrons in which the spurious signals are reduced to a minimum. An improved cathode struc ture is provided in which the shot noises are suppressed by means of a space charge, and random interception of electrons, by a limiting aperture, is reduced substantially.

It is an object of the invention to provide an improved pick up tube.

Another object of the invention is to provide an improved image orthicon.

Another object of the invention is to provide an image orthicon having increased sensitivity at low light levels.

Another object of the invention is to provide an improved electron gun structure for a pick up tube.

Another object of the invention is to provide improved means for forming an electron beam in a picli up tube.

Another object of the invention is to provide novel means for reducing spurious signals in a pick up tube.

The above and other objects and features of the invention will appear more fully hereinafter from a con-f sidcration of the following description taken in connection with the accompanying drawing, wherein o-ne embodiment is illustrated by way 'of example.

In the drawing:

Figure l is a diagrammatical representation of a pick up tube embodying the invention.

Figure 2 is an enlargedsectional view of the cathode of Figure l.

Referring now to the drawing wherein an image orthicon is indicated generally by the numeral Zi which may be of the general type as described in the Proceeding of The institute f Radio Engineers, volume 34, pages 4124-432 (1946). The device 1 has a glass envelope 2 which has a metallic coating 3 on the interior thereof. Surrounding the envelope Z are focusing coil 4, horizontal and vertical deecting coils 5, and alignment coil 6. inasmuch as the connections to the several elements are conventional, they have been omitted for simplicity.

A photo-cathode 7 is located within the envelope 2 adjacent one end thereof. Spaccd from the photo-cathode 7 is a target screen 8 and target 9 respectively. A decelerating grid iti is provided adjacent the side of the target away from ther photo-cathode.

Adjacent the opposite end of the envelope 2 are the electron gun lll, multiplier l2, and electrode 13. The electron gun ill includes a cathode 14. An enlarged sectional view is shown in Figure 2.

The cathode )la has a cylindrical wall section 1S, a bottom wall 1.6 and a top wall i7. The top 17 has an aperture it concentric with the cylindrical wall 15. The internal surfaces of the wall i5 and bottom 16 are coated with an oxide coating 19 of a conventional type as used for cathode coatings in electron tubes. The top 17 is uncoated. The wall l5 and bottom 16 may be made of a conventional cathode metal, such as for example nickel. The top i7 is of a material which has the property, that when heated to the temperatures normally used for an oxide cathode, will reduce the alkaline earth oxides absorbing the oxygen and re-evaporating the volatile alkaline earth metals, for example titanium. A heater 20 is provided for the cathode 14.

An accelerator grid 21 may be positioned between the cathode i4 and dynode grid 22. The grid 231 and 22 are provided with apertures concentric with the aperture 18 and slightly larger in diameter. The grid 22 forms the first stage of the multiplier l2.

ln operation, electrons are emitted from the coated wall l5 and bottom 16 and form a space charge cloud of electrons within the cathode i4. An electron beam, originating from the electron cloud, will be formed by the aperture l. As the titanium surface remains clean and is a relatively poor electron emitter, any electrons which form the electron beam originate in the space cloud within the cathode 14. Thus the shot noise in the electron beam will be suppressed by the action of the space charge cloud within the cathode. By making the apertures in the accelerating and dynode grids larger than the aperture in the cathode, random interception of electrons by the edge of the apertures, which produce shot noises, will be reduced to a minimum. The remainder of the operation will be somewhat similar to that set forth in the Proceeding of the i. R. E., vol. 34, pages 424432 (1946) and will be omitted here for brevity.

Although only one embodiment of the invention has been illustrated and described, various changes in the form and relative arrangement of the parts, which will now appear to those skilled in the art, may be made without departing from the scope of the invention.

What is claimed is:

A cathode for an electron discharge device requiring a pencil-like beam of electrons from said cathode, said cathode comprising a cylindrical body having a side wall, a bottoni wall and a top wall, a circular opening in said top wall concentric with said side wall and small relative to said top wall, a coating of cathode emissive material on said bottom and side walls, and a heater for said cathode said bottom and side Walls being of a material having good electron emissive characteristics, and said top wall being of titanium and having relatively poor electron emissive characteristics.

References Cited in the tile of this patent UNITED STATES PATENTS 2,677,623 Delrieu Mayv 4, 1954 OTHER REFERENCES Kumpfer: The Hollow Cathode In Cylindrical Geometry, Convention Record of the LRE., 1954 National Convention, part 3, Electron Devices and Component Parts, pp. 66 to 69.

Poole: Emission From Hollow Cathodes, Jour. of Appl. Physics, vol. 26, pp. 1176-1179, Sept. 1955, 

